Significant seasonal variation of blood cholesterol levels has been shown in a number of cross-sectional studies, with important implications for national screening and therapeutic guidelines. However, the phenomenon has not been well studied: little is known about etiology or about seasonal variation of lipid subfractions. The primary aim of this proposal, therefore, is to further describe and delineate in a prospective study the nature and causes of seasonal variation of blood lipid levels in the general population. Specifically we will: l. Assess the magnitude and timing of the seasonal effect for total cholesterol and the important lipoprotein subfractions (low-density lipoprotein, high-density lipoprotein, triglycerides, apolipoproteins AI and B, and lipoprotein (a)) in both sexes and at different ages; 2. Identify and quantify the effects of the main factors determining the variation; 3. Assess the seasonal variation of other blood elements thought to play a role in the development of coronary heart disease, including hemostatic factors and antioxidants; 4. Explore the implications of the phenomenon for public health policy. The study population will be 600 randomly selected adults of both sexes who receive their medical care at the Fallon Clinic, a large health maintenance organization in central Massachusetts. Baseline and serial measures repeated quarterly for one year will be carried out. These will include measures of: serum lipids and lipoproteins; antioxidant vitamins; platelets and hemostatic factors; demographic factors; weight, waist-hip ratio, height, BMI and blood pressure; dietary intake; physical activity; light exposure; and psychologic factors. Meteorologic data will also be collected. Data will be collected by written questionnaire and by the use of computer-assisted evening telephone interviews (CATI), which will be used for the collection of dietary, physical activity and individual light-exposure and other lifestyle data. Activity and light data will be verified in a subset of 80 patients by the use of a monitoring device. Analyses will include: 1) methods to describe the seasonal variation in dietary, physical activity, psychologic, and light exposure data; and 2) repeated measures linear regression models to determine the roles of specific dietary, physical activity, psychologic, and light-exposure related covariates in explaining variability in blood lipids, antioxidant vitamins, and hemostatic factors. We will also explore the screening and therapeutic implications of our findings for both the individual practitioner and for national guidelines.